Multiple print() and/or println() calls to write each record. There is no advantage in writing the whole record at once. At the end, each character is shifted out individually. You might as well buffer them that way.

sprintf will put your values ("day(t),month(t),year(t)" here) into a specified format ("%.2d,%.2d,%.4d"--google sprintf for information and examples on formatting), and save that into the designated array (dataString).

For this to work, you have to know the maximum length of one line of data, and set the length of the char array to at least one more than that (to allow for the null that will be added at the end). However, this will print your data using two lines of code (the sprintf to build your output string, and the println to actually print it), rather than five to separately print the day, month, year, and commas between them. The code savings increases as you decide to log more things. Obviously you can use any data you want; I'm just showing date using the Time library, but anything else could be stored instead.